498
��SCIENCE.
��[Vol.. 1
��verj' large cajiflcity. placed low eiioiiijli in the CrotoD valley to iucrease to 3G1.!K"2 square miles the available area of the watershed of CroLon River. This resonoir is to \m\v. a caiiacity of 3,200.000.000 galloiis. — a bodv of wai«r which would cover 9,400 acres tea feet deep.
The dam which is to form this reservoir (the Quaker-bridge dam), 178 feet bigh above the bed of the river, is to be built of solid ma- sonrj', and the water behind it is to be 171 feet^deep. Ae the foundations of the dam most be extended to the bed-rock, a distance of nearly 1 00 feet below the bed of the stream, thfe total height of the masonry atriicture will consequently be not far from 300 feet for a length of 400 feet in the deeper portion of the valley. On both aides of this deeper i>ot- tion the rook-bottom lises gradually, and the total length of the dam is to be about 1,300 feet.
The height mentioned fur a masonry dam 18' UDpi'ecedeuted; and the strains whiib will be transmitted to the base of the structure by the combined action of its own weight and of the watcr-preaeure are such as to require in the design a departure from the methods used ind reeommonded liy the engineering author- ities who have studied the question of high masonry dnms of leaser magnitude. The width of the dam at its bate, although not ftllly decided upon, is to be more than 200 feet.
The question of providing an overflow to liberate the surplus water which must he wasted over the dam is happily and economi- cally solved by natuiv, which has provided in the immediate vicinity a depression in the ifock-formation, of the required elevation and form for the safe disposal of the freshets.
The new aqueduct starts from a point near the present Crolon dam. and follows a general southerly direction towards the city, to 135Ui Street, with a length of nearly ihii-ty-one miles. For the remaining distance, from IS.^th Street to the reservoir in Centra! Pork (two and one- third miles), the water is to be conveyed in pipes. Hailom River is crosseii by means of an inverted siphon 150 feet below the water anrface.
With the exception of three points where it comes to the surface of the ground for short distances, the aqueduct is to be wholly in tun- nel; and from the indications furnished by the topographical character of Ihe country, and by numerous borings made with the diamond drill, it is probable that the excavation is to be, almost for the whole length of the aqueduct,
��tOBB
4
��in solid rock. It is expected that n Inr^ |)ortion of the tunnel excavatioc is lo be lii with masonry; but, wherever the t-linracbv the rock is such that it can remn without danger of falls, the mnsonrv is to btf<' dispensed with. If the line of work tiad been so iofiilod as to allow of the coustl-iiciion of the aqueduct in open trenches of modt-mlr depth, it would have been much longer, owing to the necessity of following the cont«iiis of the land; and it would have passed alon^ ibi? east shore of Hudson River, Ihrougli ibiikk settled commnnities, where the land -da in ages would have been much higher. The tunnel presents also the important advantage of being almost wholly safe from the attacks of a mob or of a military foe.
From Croton dam to a point south of and near the boundary of the cities of Vonkers and New York, the aqueduct has a inaximniii flowing capacity of 320.000.000 gallons per day; it is 13.6 feet high and 13,6 ft-et wide: and its section is that of a semloirctilar arch, suppoited on slightly concave aides, llio bottOB being formed by a flat inverted arch.
At the point just mentioned, where it is pected that a large distributing reservoir is be built to supply the annexed district, and where consequently a i>ortion of the supply is to be diverted, the flowing capacity of the aqueduct is reduced to 250,000.000 gallons per day, and its form is circular, with a diameter of twelve feet three inches.
This part of the aqueduct, over six and a half miles in length, including the inverted siphon under Harlem River, is to be heavily lined with masonry; and, owing to the tDiuf- ficient elevation of the land, it is depressed to a considerable depth, presenting the peculiar. and to a certain extent experimental, feature of a masoniy channel built in solid rock, and subject to a considerable internal water-]>ress- iire. lis prototype, the tunnel under Dor- chester liay. which conveys the sewage of Boston lo Moon Island, has been in successful operation for more than a year, but under somewhat dilTerent conditions of location, size, and pressures.
For the purposes of constmotion and of future maintenance of the aqueduct, thirty- two shafts are provided, of various depths, the greatest l>eing ZhQ feet. Twenty-four shafts are uu<ler construction, twelve of which are already i-ompleted, or nearly so.
Sis extensive gate-chambers are to be con- structed, in connection with the aqueduct, for the puriKise of emptying it when necessari". and of regulating the flow of water from the
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